Shaped catalysts or catalyst precursors are known in the art and have been described, for example in European Patent EP-0,127,220.
The preparation of hydrocarbons from a gaseous mixture comprising carbon monoxide and hydrogen by contacting the mixture with a catalyst at elevated temperature and pressure is known in the literature as the Fischer-Tropsch synthesis.
Catalysts used in the Fischer-Tropsch synthesis often comprise one or more metals from Group VIII of the Periodic Table of the Elements, optionally in combination with one or more metal oxides and/or other metals as promoters.
It is desirable to employ a highly efficient catalyst. In terms of the Fischer-Tropsch process, a highly efficient catalyst is one which exhibits not only a high level of activity for the conversion of carbon monoxide and hydrogen to hydrocarbons, but also a high degree of selectivity to higher molecular hydrocarbons, in particular C5 hydrocarbons and larger, henceforth referred to as “C5+ hydrocarbons”. Preferably, the degree of branching in the C5+ hydrocarbons should be low. It is taught in the prior art that the efficiency of a catalyst in general increases as the size of the catalyst particle decreases. Further, catalysts should show a high stability, i.e. deactivation should be very low.
The Fischer-Tropsch synthesis may be carried out using a variety of reaction regimes, for example a fluidized bed regime or a slurry bed regime. When using a process employing a fixed bed of catalyst particles, a major consideration in the design of the process is the pressure drop through the catalyst bed. It is most desirable that the pressure drop should be as low as possible. However, it is well reported in the art that, for a given shape of catalyst particles, as the size of the catalyst particles in a fixed bed is reduced, there is a corresponding increase in pressure drop through the catalyst bed. Thus, there exists a conflict in the design of fixed catalysts beds when trying to achieve a satisfactory level of catalyst efficiency while keeping the pressure drop through the bed to a minimum.
In addition to the above, the catalyst particles should be sufficiently strong to avoid undesired attrition and/or breakage. Especially in fixed beds the bulk crush strength should be (very) high, as beds are used in commercial reactors of up to 15 meters in height. Especially at the lower end of the bed the strength of the catalyst particles plays an important role. This is an additional complication in designing further improved catalyst particles.
A still further complicating element is the manufacturing process of catalyst particles. There is a need for a fast, relatively inexpensive and suitable manufacturing process which will enable the production of catalyst particles in large quantities. One example of such a commercially available manufacturing process is an extrusion process.
Accordingly, there exists a need for a catalyst or catalyst precursor comprising a Group VIII element and optionally a promoter selected from the elements of Group IIA, Group IIIB, Group IVB, Group VB, Group VIB or Group VIIB of the Periodic Table of the Elements which catalyst displays a high activity and selectivity in the Fischer-Tropic synthesis process, while keeping the pressure drop in the fixed bed as low as possible and displaying a high crush strength and stability.
In the past a tremendous amount of work has been devoted to the development of particles, in particular catalytically active particles, for many different processes. There has also been a considerable effort to try to understand the advantages and sometimes disadvantages of effects of shape when deviating from conventional shapes such as pellets, rods, spheres and cylinders for use in catalytic as well as non-catalytic duties.
Examples of further well-known shapes are rings, cloverleafs, dumbells and C-shaped particles. Considerable efforts have been devoted to the so-called “polylobal”-shaped particles. Many commercial catalysts are available in TL (Trilobe) or QL (Quadrulobe) form. They serve as alternatives to the conventional cylindrical shape and often provide advantages because of their increased surface-to-volume ratio, which results in a smaller effective particle size, thus providing a more active catalyst.
A variety of shapes and designs of catalyst particles for use in the fixed bed operation of the Fischer-Tropsch synthesis have been proposed. Thus, EP-0,428,223 discloses that the catalyst particles may be in the form of cylinders; hollow cylinders, for example cylinders having a central hollow space which has a radius of between 0.1 and 0.4 of the radius of the cylinder; straight or rifled (twisted) trilobes; or one of the other forms disclosed in U.S. Pat. No. 4,028,221. Trilobe extrudates are said to be preferred.
EP-0,218,147 discloses a helical lobed, polylobal extrudate particle having the outline shape of three or four strands helically wound about the axis of extrusion along the length of the particle and its use as a catalyst or catalyst support, in particular as a catalyst or catalyst support in hydrotreating operations. The helical shape of the catalyst is said to reduce the pressure drop across fixed bed reactors through which liquid and/or gas reactants are passed. In this way, smaller catalyst particles can be employed in a given reactor design to meet the pressure drop requirements.
In EP-0,220,933, it is described that the shape of quadrulobe-type catalysts is important, in particular with respect to a phenomenon known as pressure drop. From the experimental evidence provided it appears that asymmetric quadrulobes suffer less from pressure drop than the closely related symmetrical quadrulobes. The asymmetrically shaped particles are described in EP-0,220,933 by way of each pair of protrusions being separated by a channel which is narrower than the protrusions to prevent entry thereinto by the protrusions of an adjacent particle. It is taught in EP-0,220,933 that the shape of the particles prevents them from “packing” in a bed causing the overall bulk density of the catalyst bed to be low.
Since many of the findings in the art are conflicting and pressure drop problems continue to be in existence, especially when surface-to-volume ratios are increased by reducing particle size, there is still considerable room to search for alternative shapes of catalytically active particles which would diminish or even prevent such problems.
It would be useful to find specifically shaped catalyst particles or catalyst precursor particles that offered unexpected and sizeable advantages compared with conventional “trilobal” catalyst particles, especially when used in mass transfer or diffusion limited reactions in fixed-bed reactors, for instance as catalysts in the Fisher-Tropsch process.